The present invention relates to polymeric materials formulated to control optical dispersion, and more particularly, to the use of these dispersion-controlled polymeric formulations in broadband fiber optic device applications.
Dense wavelength division multiplexed (DWDM) optical networks increase their transmission capacity by employing multiple co-propagating, discrete, wavelength channels, each carrying independent data streams. Broadband fiber optic devices, such as variable attenuators, couplers, and switches having a controllable spectral response, are critical components of DWDM systems. Currently, DWDM systems operate in the 1550 nm spectral region because of the availability of optical amplifiers containing erbium-doped optical fibers. However, as amplifier technology develops, and capacity demands increase, DWDM systems are expected to expand their spectral extent and increase their channel density.
Optical power, as it propagates in a single-mode optical fiber, or any other waveguide or bulk material, experiences dispersion, i.e. differing wavelengths propagate at different speeds. In an optical fiber, modal extent and phase velocity are affected by both the dispersion of the material (material dispersion) and the dispersion of the waveguide (waveguide dispersion) causing the light to pass through at different speeds. Thus, across a given wavelength region, differences between the dispersions of the material and wavequide through which light propagates can result in nonuniform spectral performance of fiber-based devices.
Dispersion is often represented in terms of a material""s refractive index (n) as a function of optical wavelength (xcex), i.e. as n(xcex). In dispersive materials, the refractive index of the material changes with wavelength. The relevant parameter when describing modal dispersion or multimode distortion in optical fibers is the effective mode index, also referred to herein as xe2x80x9ceffective mode dispersionxe2x80x9d, neff(xcex), which, in simple waveguide geometries, can be calculated using the material dispersion of the fiber""s cladding and core, nclad(xcex) and ncore(xcex), respectively, and geometric parameters. Sellmeier dispersion equations for the cladding and core in a single mode optical fiber are provided by J. Gowar, in Optical Communication Systems, ch. 3, 58-77 (2d ed. 1993). For a glass fiber, the material dispersions for the cladding and core are calculated from the following Sellmeier equations (1a) and (1b), respectively, which are valid from 0.3-3.0 xcexcm:                                                         n              clad              2                        ⁡                          (                              λ                ⁡                                  [                                      μ                    ⁢                                          xe2x80x83                                        ⁢                    m                                    ]                                            )                                -          1                =                                            0.6962              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  0.0684                  2                                            )                                +                                    0.4970              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  0.1162                  2                                            )                                +                                    0.8975              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  9.8962                  2                                            )                                                          (1a)                                                                    n              core              2                        ⁡                          (                              λ                ⁡                                  [                                      μ                    ⁢                                          xe2x80x83                                        ⁢                    m                                    ]                                            )                                -          1                =                                            0.7192              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  0.0709                  2                                            )                                +                                    0.3988              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  0.1157                  2                                            )                                +                                    0.9099              ⁢                              λ                2                                                    (                                                λ                  2                                -                                  9.9093                  2                                            )                                                          (1b)            
The dispersions of the cladding, nclad(xcex), and core, ncore(xcex), and the effective mode dispersion, neff(xcex) for a silica glass optical fiber having a core with a slightly raised refractive index relative to the surrounding cladding are plotted in FIG. 1. Although all materials are dispersive to some extent, a hypothetical material exhibiting no dispersion would be represented in the graph of FIG. 1 as a horizontal line. The greater the dispersion, the steeper the slope (negative or positive). As used herein, the term xe2x80x9cdispersionxe2x80x9d refers to the slope of the line formed from a plot of a material""s change in refractive index versus change in wavelength. As can be seen from the slope of neff(xcex) in FIG. 1, a single mode optical fiber is dispersive.
Because the effective mode index is dispersive, fiber-based devices may exhibit spectrally non-uniform performance, which is undesirable for many broadband device applications. An example of this is a side-polished fiber (SPF)-based attenuator. Cargille Refractive Index Liquids, which may be coupled onto the attenuator, each have a well-characterized refractive index, nD, where subscript D denotes the Sodium D-Line wavelength (xcex=589 nm), and a well-characterized dispersion curve. As disclosed in copending commonly assigned U.S. application Ser. No. 09/026,755 entitled xe2x80x9cFiber Optic Attenuators and Attenuation Systemsxe2x80x9d, the disclosure of which is incorporated herein by reference, placing a coupling oil (nD=1.456 at 27.9xc2x0 C.) on a SPF coupler induces power loss (attenuation). FIG. 2a is a plot of attenuation (optical energy transmission) in decibels versus wavelength (1520-1580 nm) for a SPF coupler having a 95% polished cladding level. As shown in FIG. 2a, the attenuation is not uniform across the spectral region. This spectral nonuniformity is observed because the dispersion of the oil, noil(xcex), which is calculated from equation (2) as                                           n            oil                    ⁡                      (                          λ              ⁡                              [                Å                ]                                      )                          =                  1.44418          +                      401173.2                          λ              2                                +                                    3.18914              ⁢                              .10                11                                                    λ              4                                -                                    3.89              ·                              10                                  -                  4                                                      ⁢                          (                                                T                  ⁡                                      [                                          xc2x0                      ⁢                                              xe2x80x83                                            ⁢                                              C                        .                                                              ]                                                  -                25                            )                                                          (        2        )            
(where T is the temperature) is mismatched to that of the fiber, neff(xcex). This dispersion mismatch is depicted in FIG. 2b, where the slope of noil(xcex) differs from that of neff(xcex). By contrast, if the dispersion of the oil matched that of the fiber, the graphic representations of the corresponding dispersions would be approximately parallel, and the attenuation would be almost constant or substantially uniform across the wavelength band with only small variations being observed.
As disclosed in the aforementioned U.S. application Ser. No. 09/026,755, certain organic polymers having an index of refraction close to that of the fiber can be applied to the exposed surface of a SPF optic for use in variable optical attenuators (as described below). Such polymers exhibit a change in refractive index proportional to a change in temperature. OPTI-CLAD(copyright) 145, which is available from Optical Polymer Research, Inc. is an example of such a polymer. Although the refractive index of such organic polymer materials can be altered at a given wavelength to match that of the fiber, the use of known polymers is limited in broadband applications because of the dispersion mismatch between the polymer and the fiber across the wavelength band of interest.
Control over the spectral response of a broadband fiber optic device can be strongly dependent on the dispersion mismatch between the fiber and any coupling materials present. Therefore, polymer formulations are desirable that would allow not only control of the refractive index of the polymer overlying the optical fiber, but also control of the dispersion properties of the polymer. Such dispersion control would permit correction of the polymer""s dispersion to substantially match that of the fiber and, alternatively, would allow the dispersion of the polymer to be controllably altered from that of the fiber. Such dispersion controllable materials would be useful in broadband applications, such as in the 1500-1600 nm region, where control of spectral response is important. In addition, dispersion controllable polymer materials would be useful in the fabrication of many broadband fiber optic devices, such as variable optical attenuators (VOAs), couplers, and switches.
The present invention meets the aforementioned needs and is based on the unexpected discovery that certain polar polyolefin polymers doped with infrared absorbing dyes having an absorption maximum from 900 to 1200 nm can be formulated to correct or control dispersion mismatch between the polymer and a fiber optic. The dye additives are used to control the dispersion from almost no difference in dispersion between the polymer composition and the fiber optic to very strong differences in dispersion. In addition, the refractive index of the novel polyolefin compositions of the present invention can be controlled and can be altered to match or differ from that of the optical fiber. The novel dispersion-controlled polymer compositions of the present invention are particularly useful in the fabrication of spectrally uniform fiber optic devices such as VOAs, couplers, and switches for use in the 1500-1600 nm region.
Accordingly, in one aspect, the present invention is a dispersion-controlled polymer composition comprising:
(a) from about 0.2 to about 4% by weight of an infrared absorbing dye having an absorption maximum from about 900 to about 1200 nm; and
(b) from about 96.0 to about 99.8% by weight of a polar olefin copolymer comprising monomeric units derived from two or more polar olefins having an ester, benzene, or halogen substituent attached thereto.
The polar olefin copolymer preferably has a molecular weight from about 1,500 to about 100,000 g/mole, with an upper limit of about 50,000 g/mole being more preferable, and an upper limit of about 5,500 g/mole being the most preferable.
The infrared absorbing dye is preferably:
(8-((3-((6,7-dihydro-2,4-diphenyl-5H-1-benzopyran-8-yl)methylene)-2-phenyl-1-cyclohexen-1-yl)methylene)-5,6,7,8-tetrahydro-2,4-diphenyl-1-benzopyrylium tetrafluoroborate or a metal complex dye having the general formula bis[1,2-[(4-alkyl1 alkyl2 amino)phenyl]-1,2-ethylenedithiolate]Met. Alkyl1 and alkyl2 are each independently lower alkyls containing 2 to 8 carbon atoms. In addition, alkyl1 may differ from or may be the same as alkyl2. Met is a Group IIIB metal, preferably nickel, palladium or platinum. The most preferable metal complex dyes include bis[1,2-(4-dibutylaminophenyl)-1,2-ethylenedithiolate]nickel; bis[1,2-[4-(ethyl heptyl amino)phenyl]-1,2-ethylenedithiolate]nickel; bis[1,2-(4-dibutylaminophenyl)-1,2-ethylenedithiolate]platinum; and bis[1,2-[4-(ethyl heptyl amino)phenyl]-1,2-ethylenedithiolate]platinum.
The polar olefins, from which the monomeric units of the copolymer are derived, are preferably selected from, but not limited to:
tetrafluoropropyl acrylate, tetrafluoropropyhnethacrylate, butyl acrylate, hexyl acrylate, trifluoroethyl methacrylate, lauryl acrylate, pentafluorostyrene, pentafluorophenyl acrylate, methyl acrylate, N, N-dimethylacrylamide, pentafluorophenyl methacrylate, methyl methacrylate, and vinylidene chloride.
In another aspect, the present invention is a method for controlling dispersion in the aforementioned novel polymer composition across a wavelength band, the most preferable wavelength band being from about 1500 nm to about 1600 nm. The first step of the method is providing a portion of an optical fiber through which optical energy can propagate. The portion of the optical fiber has material removed from it, thereby exposing a surface thereof The portion of the optical fiber has an effective mode refractive index at each wavelength of the wavelength band and an effective mode dispersion across the wavelength band.
In the second step of the method, the polymer composition is formed over the exposed surface of the portion of the optical fiber. The polymer composition has a material refractive index at each wavelength of the wavelength band and a material dispersion across the wavelength band. By altering the amount of the infrared absorbing dye present in the polymer composition, the material dispersion across the wavelength band is controlled to substantially match or mismatch the effective mode dispersion of the optical fiber. Also, the amount of dye, as well as the particular polyolefin contained in the polymer composition, controls the material refractive index at each wavelength. For substantial broadband dispersion matching, two conditions must be met: 1) the material refractive index of the polymer composition must be substantially the same as the effective mode refractive index; and 2) the change in refractive index across the wavelength band of interest (i.e. slope) must be substantially the same for both the optical fiber and the polymer composition. For example, a polymer composition which is substantially dispersion-matched with an optical fiber preferably has a material refractive index which is controlled to lie within about 0.5% of the effective mode refractive index, more preferably within about 0.2%, and most preferably within about 0.15%. In addition, to substantially match the dispersion of the fiber optic, the change in material refractive index with wavelength is, for example, preferably within about 25% of the change in effective mode refractive index across the wavelength band of interest.
The method may optionally include the step of altering the material refractive index to a desirable refractive index without changing the material dispersion. This may be accomplished by controllably changing the temperature of the polymer composition formed on the surface of the optical fiber.
In another aspect, the present invention is an optical device comprising a portion of an optical fiber having material removed therefrom, thereby exposing a surface thereof, and the novel polymer composition formed over the exposed surface of the optical fiber. Optical energy or light transmitted through the fiber can propagate through or be extracted from the exposed surface. The spectral response of the optical device across a wavelength band of interest may be controlled by controlling the material dispersion relative to the effective mode dispersion. For uniform spectral response, the material dispersion substantially matches the effective mode dispersion.
In addition, the optical device may optionally include a temperature controlling circuit coupled to the polymer composition, wherein the temperature controlling circuit provides a controllable stimulus to the polymer composition to change the temperature thereof The temperature of the polymer composition affects the material refractive index without altering the material dispersion or the spectral response of the device.
Based on the present invention, dispersion mismatch between an overlying polymer and an optical fiber can be controlled and corrected, if desired. This control is possible through modification of the composition of the polymer overlay using an infrared absorbing dye, as described above. In addition, improvement in spectral performance across a wavelength band is possible. The availability of the present dispersion and refractive index controlled polymer compositions can be used to develop novel fiber optical devices, such as attenuators, switches, and couplers.